Basin structures for swimming pools are known which are formed in situ out of concrete. It has been found to be impossible to make a rugged swimming pool out of concrete slabs or the like because the conventional joints inevitably leak. One of the principal disadvantages of the concrete pool is that it must be repainted yearly. This is a difficult and expensive operation which can only be carried out in clear and warm weather, so that the swimming pool is inevitably unusable during a period when its use is most desired.
It is also known to make a swimming pool out of a plurality of metal, usually steel, plates which are welded together. The disadvantage of such a structure is that it is very sensitive to heat deformation. Heat generated by welding frequently deforms the plates when they are connected together and, when part of the pool is in shadow and part in sunlight the structuure frequently deforms to the point of breaking. In addition it is impossible to make such a structure in the colder months of the year, as the inevitable changes in dimensions that come with change of seasons will cause the various connecting pipes and the like to be ripped loose.
It is also known to make swimming pools out of relatively porous bases which are not watertight themselves, but which are covered by a watertight synthetic-resin sheet. Although such a structure is relatively inexpensive to manufacture but the sheet is frequently fragile, and once damaged the structure leaks readily.
It is also known to use relatively strong synthetic-resin sheets, usually reinforced with glass fiber, and to bolt these sheets together. The bolting is generally carried out on the outside of the basin structure so that when completed the pool has a smooth interior. The principal disadvantage of such an arrangement is that the bolts, which are buried, frequently are completely destroyed by corrosion, allowing the joints to separate and the pool to leak. In addition it can be impossible to tighten the joints in such pool, as the bolts are completely inaccessible in the finished structure.
In order to keep the water in such pools fresh and healthy it is necessary that it be continuously passed through a filter, usually with the addition of a predetermined quantity of chlorine to the water as it is circulated. Nowadays this flow is generally from one side of the pool to another. In addition it is known to inject water at the bottom of the pool and withdraw it from an overflow rim at the upper edge of the pool.
In recent years a new system has become popular wherein the water is injected in the base or floor of the pool at several locations through holes which effectively form an upwardly directed jet of purified water in the pool. Such a system, although more efficient than any of the abovementioned arrangements, has the disadvantage that in reality the water exchange is only a fraction of that which it theoretically appears to be, as the upwardly flowing jet merely forms an inverted cone of fresh water in the pool, leaving many zones virtually unaffected by the water flow. In addition the use of such systems creates the considerable danger that a leak will appear in the water-feed lines below the swimming pool or at the inlet holes. Such a leak can have disastrous consequences for the pool because it is extremely difficult to plug.